RetroRacer

Discussion in 'Scratch Built' started by Rockhaus, May 20, 2019.

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  1. Oct 15, 2019 #31

    jqavins

    jqavins

    jqavins

    Joseph Avins TRF Supporter

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    The mass limit wasn't the point. "About the same impulse" means about the same altitude, while "shorter burn" means higher thrust, thus better speed off the rail.
     
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  2. Oct 16, 2019 #32

    Rockhaus

    Rockhaus

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    Yeah, it's really getting up to a safe speed quickly that I'm most concerned with. The F's peak early in the burn, so that helps. I'm not way off from the 5:1 recommended ratio, and this ship will have a fair amount of inertia coming off the rail. Again, I'll need all seven.
     
  3. Oct 16, 2019 #33

    Nytrunner

    Nytrunner

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    That's the right kind of research!

    The thrust during that first quarter second of the burn is much more important than the average thrust.

    Just wait till you find progressive motors that don't reach average thrust until a second or more into the burn
     
  4. Oct 16, 2019 #34

    jqavins

    jqavins

    jqavins

    Joseph Avins TRF Supporter

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    Certainly true, but there's one aspect I've struggled with. The 5:1 thumb rule is for average thrust. Is there a handy rule of thumb for first-quarter-second thrust? It obviously needs to be more than 5:1, and an actual simulation is obviously better than any rule of thumb. Still, thumb rules are really convenient when they work, so it would be nice to have one.

    P.S. It would be easy enough to calculate how many gees (the same as thrust to mass ratio) it takes to reach a given speed within a given length, but reality is rarely as clean as such an easy calculation.
     
  5. Oct 16, 2019 #35

    Nytrunner

    Nytrunner

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    Because that is the number stamped on the package, and the number fliers write on their cards. Average Jim Rocketeer can use that number most of the time and be fine.

    The 5:1 approximation works great during the initial portion of the burn since that's the time it's actually on the rail. For lots of motors, there isnt too much difference.

    The real key is rail exit velocity as you stated. That should definitely be accounted for during design and planning, especially if an RSO has doubts. Produce the data for your rocket and motor, and you have a better position than "this motor should work....."

    The safety rule via NAR is 3:1. And that was a wise decision because there are motors out there that have serious thrust spikes far outpacing the "average" thrust in the motor name. If 5:1 was a safety rule, some flights on those motors would be disallowed based on avg thrust, when in reality they'd be smooth sailing.
     
  6. Oct 16, 2019 #36

    jqavins

    jqavins

    jqavins

    Joseph Avins TRF Supporter

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    Yes, I'm aware.

    The issue is that the two statements above contradict each other. In fact, most BP motors have such an early spike. What I'm saying is that it would be handy to be able to say "It doesn't meet the 5:1 average (named) thrust. But look, it passes the X:1 early thrust" without having to resort to the otherwise superior simulation results. One approach would be to look at that spike on typical BP motors; is it 3 times the average? Then use 15:1. Is it double? Then use 10:1. Then mostly the same BP motors pass or don't pass, and such a rule could be applied, as a a quick check only, to motors with different profiles.
     
  7. Oct 16, 2019 #37

    Nytrunner

    Nytrunner

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    I can't agree.

    Where's the contradiction? "Lots of motors" doesn't contradict the (some) motors whose average thrust is significantly different from the thrust on the rail. Yes, those can be considered edge cases, but they bite you occasionally.
    I've personally witnessed a few folks use smoky motors or similar with low initial thrust because "the average thrust was 5:1" flop off the rail and cruise missile (or worse)
    Of course none of this matters if you like the punchy motors where the avg thrust is 10 times the weight of your rocket.

    The spike-to-average thrust ratio isn't constant. Creating such a rule would only lead to more confusion down the road.
    If you can, simulate your exact rocket with the motor you have planned to fly. That gives you a good rail speed.
    If you're buying motors on the field and can't sim, peak at the thrust curve and use that to show that the motor has enough oomph off the pad even though the avg isn't quite 5:1 (we all like low-an-slow right?)


    It's a hobby, but it's still rocket science. Rules of thumb can be ok in a pinch, but they have limitations


    Also, sincere apologies to Rockhaus, we've gone and muddled up your thread. I'll stop
     
  8. Oct 16, 2019 #38

    jqavins

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    Joseph Avins TRF Supporter

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    Contradict or not, I guess it's a matter of perspective. I"m thinking of BP motors, which (after all) is that Rockhaus is proposing to use. Most of those have the spike. Otherwise, yes you're right, "contradict" was an overstatement on my part.

    Other than that we're saying mostly the same things. "Rules of thumb are OK in a pinch" and "If you're buying motors on the field and can't sim, peak at the thrust curve and use that..." being the core. Looking on the field at the peak thrust, there should be a rule of thumb, and that's not 5:1.
     
  9. Oct 16, 2019 #39

    Nytrunner

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    Don't know about that either. Seems to be working for me so far.
    If a flier wanted 7:1 to be uber-safe, I wouldn't argue it
     
  10. Oct 20, 2019 #40

    Rockhaus

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    Glad to have fired up some good discussions on this. I'm a big proponent of doing our homework (especially for the big ones). It just takes that one to go badly. As a friend of mine used to tell his kids: "If you're going to do something stupid, at least be smart about it."

    Figures there'd be 25-30 mph winds with even higher gusts forecasted for launch day tomorrow. Rats.
     
  11. Oct 20, 2019 #41

    Nytrunner

    Nytrunner

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    Ouch. That's a bit stiff for my blood
     
  12. Oct 20, 2019 #42

    BABAR

    BABAR

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    A surgeon giving a lecture said

    The first 5 years is learning HOW to operate

    The next 5 years is learning WHEN to operate.

    The rest of the career is learning when NOT to operate.

    It takes a wise Rocketeer to know when NOT to launch the rocket!
     
  13. Oct 21, 2019 #43

    jqavins

    jqavins

    jqavins

    Joseph Avins TRF Supporter

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    Reminds me of what I told my nephews. "I've known your father for a long, long time, so I know the day will come when you do stupid things with flammable liquids. It's in your blood. For goodness sake, use kerosene. Don't use gasoline, use kerosene." I then proceeded, as safely as possible, to demonstrate why. (And no one was hurt.)
     

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